Organic thin film transistor

ABSTRACT

A thin film transistor comprising at least three terminals consisting of a gate electrode, a source electrode and a drain electrode; an insulator layer and an organic semiconductor layer on a substrate, which controls its electric current flowing between the source and the drain by applying a electric voltage across the gate electrode, wherein the organic semiconductor layer comprises a styryl derivative having a styryl structure expressed by C 6 H 5 —CH═CH—C 6 H 5 , or a distyryl structure expressed by C 6 H 5 —CH═CH—C 6 H 5 —CH═CH—C 6 H 5  each without molecular weight distribution. The transistor has a fast response speed (driving speed), and further, achieves a large On/Off ratio getting an enhanced performance as a transistor.

TECHNICAL FIELD

The present invention relates to an organic thin film transistor havingan organic semiconductor layer. Particularly, the present inventionrelates to an organic thin film transistor comprising a compound havinghigh mobility and capable of high-speed operation.

BACKGROUND ART

Thin film transistors (TFT) are broadly used as switching elements fordisplay devices such as liquid crystal display, etc. A cross sectionalstructure of a typical conventional TFT is shown in FIG. 10. As shown inFIG. 10, TFT comprises a gate electrode and an insulator layer in thisorder on the substrate, and further, comprises a source electrode and adrain electrode formed above the insulator layer having a predetermineddistance between them. Over the insulator layer exposing between theelectrodes, a conventional semiconductor layer is formed having partialsurfaces of each electrodes. In TFT with such a structure, thesemiconductor layer forms a channel region and an electric currentflowing between the source electrode and the drain electrode iscontrolled by a voltage applied to the gate electrode resultantlycausing an On-Off operation.

Conventionally, TFTs were fabricated employing amorphous silicon orpolycrystalline silicon, however, there were problems that makingscreens large in display devices or so with the use of TFTs isaccompanied by significantly soaring in manufacturing cost because achemical vapor deposition (CVD) equipment used for the preparation ofTFTs employing the silicon is very expensive. Further, because afilm-forming process of the amorphous silicon or the polycrystallinesilicon is carried out under an extremely high temperature, causing alimitation in kinds of the material employable as a substrate for TFT,there was the problem that a lightweight polymer film substrate or so isunemployable.

For the purpose of overcoming such a problem, a TFT with the use of anorganic substance replacing the amorphous silicon or the polycrystallinesilicon is proposed. With regard to the film-forming process forfabricating a TFT employing organic substances, a vacuum vapordeposition process or a wet-coating process is well known. Thosefilm-forming processes enable not only to realize making screens largein display devices while suppressing soaring in manufacturing cost butalso to relatively reduce a process temperature required forfilm-forming. Accordingly, a practical use of the TFT employing anorganic substance is highly expected because of an advantage in littlelimitation in a selection of material for a substrate and as a result, alarge number of report about TFT employing an organic substance arepublished. Examples of the report include Non-Patent Literatures 1 to 19below.

Further, with regard to the organic substance employable in an organiccompound layer of TFT, a multimer such as conjugate polymer or thiophene(refer to Patent Literatures 1 to 5 below, etc.); metallophthalocyaninecompound (refer to Patent Literature 6 below, etc.); or condensedaromatic hydrocarbon such as pentacene (refer to Patent Literatures 7and 8 below, etc.) is used singly or as a mixture in combination withanother compound each other. With regard to the materials of n-type FET,for example, Patent Literature 9 below discloses1,4,5,8-naphthalenetetracarboxyldiunhydride (NTCDA),11,11,12,12-tetracyanonaphth-2,6-quinodimethan (TCNNQD),1,4,5,8-naphthalenetetracarboxyldiimide (NTCDI), etc.; and PatentLiterature 10 below discloses phthalocyanine fluoride. Additionally,although Non-patent Literature 16 teaches that oligophenylenevinyleneexhibits transistor characteristic, it does not disclose at all whatkind of structure oligophenylenevinylene has. Because a home page ofLucent Technologies in United States of America(http://www.lucent.com/press/0902/020925.bla.html) concludes that aperson among the main authors of the Non-patent Literature 16 forgeddata and because Non-patent Literature 17 withdraws the content ofNon-patent Literature 16, the present invention never gets any influencefrom Non-patent literature 16. Further, although Non-patent Literature18 mentions about TFT property of soluble phenylenevinylene, itselectron mobility is extremely as small as about 10⁻⁵ cm²/Vs because ithas a long-chain alkyl group in its center position. Furthermore,although Non-patent Literature 19 mentions about electron mobility ofphenylenevinylenepolymer (poly-paraphenylenevinylene (PPV)), it is alsoso small as 10⁻⁴ cm²/Vs that any practical performance is not achieved.Namely, because PPV being a high molecular compound has a long mainchain structure, a turbulence in crystal structure induced from bendingor molecular weight distribution of the main chain structure reduceselectronic field-effect mobility to the small values. On the other hand,although Patent Literature 11 reports about a preparation of a TFTdevice by obliquely vapor depositing a liquid crystal compound, a TFTdevice having superior performance without depending upon the liquidcrystal compound or upon a film-forming process such as the obliquelyvapor deposition process was eagerly desired.

On the other hand, there is an organic electroluminescence (EL) deviceas a device similarly using an electric conduction. However, the organicEL device generally forces to feed charges by applying a strong electricfield of 10⁶ V/cm or greater across a thickness direction of aultra-thin film of 100 nm or thinner, whereas it is necessary for theorganic TFT to feed charges for several μm or longer with high-speedunder an electric field of 10⁵ V/cm or smaller and accordingly, anenhanced electric conductivity becomes necessary for the organicsubstance itself. Despite the above circumstances, the conventionalcompounds in the organic TFT had problems in fast response as transistorbecause its capability for moving electrons was poor, because afield-effect mobility of electron was small, and because response speedwas slow. Further, On/Off ratio was also small. The above On/Off ratiois defined as a value obtained by dividing an amount of an electriccurrent flowing between a source and a drain when some gate voltage isapplied (On) by an amount of an electric current flowing there when anygate voltage is not applied (Off). A word “On electric current” usuallymeans an amount of a (saturated) electric current at a time when theelectric current between the source and the drain saturates whileincreasing the drain voltage.

-   -   Patent Literature 1: Japanese Unexamined Patent Application        Laid-Open No. Hei 8-228034    -   Patent Literature 2: Japanese Unexamined Patent Application        Laid-Open No. Hei 8-228035    -   Patent Literature 3: Japanese Unexamined Patent Application        Laid-Open No. Hei 9-232589    -   Patent Literature 4: Japanese Unexamined Patent Application        Laid-Open No. Hei 10-125924    -   Patent Literature 5: Japanese Unexamined Patent Application        Laid-Open No. Hei 10-190001    -   Patent Literature 6: Japanese Unexamined Patent Application        Laid-Open No. 2000-174277    -   Patent Literature 7: Japanese Unexamined Patent Application        Laid-Open No. Hei 5-55568    -   Patent Literature 8: Japanese Unexamined Patent Application        Laid-Open No. 2001-94107    -   Patent Literature 9: Japanese Unexamined Patent Application        Laid-Open No. Hei 10-135481    -   Patent Literature 10: Japanese Unexamined Patent Application        Laid-Open No. Hei 11-251601    -   Patent Literature 11: Japanese Unexamined Patent Application        Laid-Open No. 2005-142233    -   Non-patent Literature 1: F. Ebisawa et al. Journal of Applied        Physics, vol. 54, p 3255; 1983    -   Non-patent Literature 2: A. Assadi et al. Applied Physics        Letter, vol. 53, p 195; 1988    -   Non-patent Literature 3: G. Guillaud et al. Chemical Physics        Letter, vol. 167, p 503; 1990    -   Non-patent Literature 4: X. Peng et al. Applied Physics Letter,        vol. 57, p 2013; 1990    -   Non-patent Literature 5: G. Horowitz et al. Synthetic Metals,        vol. 41-43, p 1127; 1991    -   Non-patent Literature 6: S. Miyauchi et al. Synthetic Metals,        vol. 41-43; 1991    -   Non-patent Literature 7: H. Fuchigami et al. Applied Physics        Letter, vol. 63, p 1372; 1993    -   Non-patent Literature 8: H. Koezuka et al. Applied Physics        Letter, vol. 62, p 1794; 1993    -   Non-patent Literature 9: F. Garnier et al. Science, vol. 265, p        1684; 1994    -   Non-patent Literature 10: A. R. Brown et al. Synthetic Metals,        vol. 68, p 65; 1994    -   Non-patent Literature 11: A. Dodabalapur et al. Science, vol.        2568, p 270; 1995    -   Non-patent Literature 12: T. Sumimoto et al. Synthetic Metals,        vol. 86, p 2259; 1997    -   Non-patent Literature 13: K. Kudo et al. Thin Solid Films, vol.        331, p 51; 1998    -   Non-patent Literature 14: K. Kudo et al. Synthetic Metals, vol.        102, p 900; 1999    -   Non-patent Literature 15: K. Kudo et al. Synthetic Metals, vol.        111-112, p 11; 2000    -   Non-patent Literature 16: Advanced Materials Vol. 13, No. 16, p        1273; 2001    -   Non-patent Literature 17: Advanced Materials Vol. 15, No. 6, p        478; 2003    -   Non-patent Literature 18: W. Geens et al. Synthetic Metals, Vol.        122, p 191; 2001    -   Non-patent Literature 19: Lay-Lay Chua et al. Nature, Vol. 434,        March 10 issue, p 194; 2005

DISCLOSURE OF THE INVENTION

In order for overcoming the above problems, an object of the presentinvention is to provide an organic thin film transistor having a fastresponse speed (driving speed), and further, with a large On/Off ratio.

As a result of intensive researches and studies to achieve the aboveobject by the present inventors, it was found that an employment of astyryl derivative with a specified structure having a styryl group(—CH═CH—C₆H₅) as a material for an organic semiconductor layer in anorganic thin film transistor enables to speed up the response speed(driving speed) resultantly completing the present invention.

Namely, the present invention provides a thin film transistor comprisingat least three terminals consisting of a gate electrode, a sourceelectrode and a drain electrode; an insulator layer and a novel organicsemiconductor layer on a substrate, which controls its electric currentflowing between the source and the drain by applying a electric voltageacross the gate electrode, wherein the organic semiconductor layercomprises a styryl derivative having a styryl structure expressed byC₆H₅—CH═CH—C₆H₅ and without molecular weight distribution. Further, thepresent invention also provides the thin film transistor wherein theorganic semiconductor layer comprises a distyryl derivative having adistyryl structure expressed by C₆H₅—CH═CH—C₆H₅—CH═CH—C₆H₅ and withoutmolecular weight distribution. Still further, the present invention alsoprovides the thin film transistor wherein the organic semiconductorlayer comprises a styryl compound represented by a general formula (a)which will be illustrated below.

The transistor became to have a fast response speed (driving speed), andfurther, achieved a large On/Off ratio getting an enhanced performanceas a transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing which illustrates one embodiment about devicestructure of an organic thin film transistor of the present invention;

FIG. 2 is a drawing which illustrates another embodiment about devicestructure of an organic thin film transistor of the present invention;

FIG. 3 is a drawing which illustrates another embodiment about devicestructure of an organic thin film transistor of the present invention;

FIG. 4 is a drawing which illustrates another embodiment about devicestructure of an organic thin film transistor of the present invention;

FIG. 5 is a drawing which illustrates still another embodiment aboutdevice structure of an organic thin film transistor of the presentinvention;

FIG. 6 is a drawing which illustrates still another embodiment aboutdevice structure of an organic thin film transistor of the presentinvention;

FIG. 7 is a drawing which illustrates a device structure of an organicthin film transistor in Examples of the present invention;

FIG. 8 is a drawing which illustrates another device structure of anorganic thin film transistor in Examples of the present invention;

FIG. 9 is a drawing which illustrates characteristic curves of anorganic thin film transistor in Examples of the present invention; and

FIG. 10 is a drawing which illustrates a device structure of a typicalconventional thin film transistor.

PREFERRED EMBODIMENTS TO CARRY OUT THE INVENTION

The present invention provides a thin film transistor comprising atleast three terminals consisting of a gate electrode, a source electrodeand a drain electrode; an insulator layer and a novel organicsemiconductor layer on a substrate, which controls its electric currentflowing between the source and the drain by applying a electric voltageacross the gate electrode, wherein the organic semiconductor layercomprises a styryl derivative having a styryl structure expressed byC₆H₅—CH═CH—C₆H₅ and without molecular weight distribution. Further, thepresent invention also provides the thin film transistor wherein theorganic semiconductor layer comprises a distyryl derivative having adistyryl structure expressed by C₆H₅—CH═CH—C₆H₅—CH═CH—C₆H₅ and withoutmolecular weight distribution.

The styryl structure of the styryl derivative in the present inventionis defined as a structural unit shown in a following formula, that mayhave a substituent on a carbon atom composing this structure andfurther, the benzene ring portion may be a polycyclic condensed ring.

The distyryl structure of the distyryl derivative in the presentinvention is defined as a structural unit shown in a following formulawhich has a bonding style of continued benzene ring, olefin, benzenering olefin and benzene ring wherein the structural unit may have asubstituent on a carbon atom composing this structure and further, thebenzene ring portion may be a polycyclic condensed ring. In the abovestructural unit, a steric position of olefin and a substituting positionof benzene ring may be anywhere. The distyryl structure strengthens amutual action between compounds and as a result, enables to obtain anenhanced current control characteristic.

It is preferable that the distyryl structure has a following structurewith a bonding position of two —CH═CH—C₆H₅ structure bonding to acentral benzene ring each exists at para positions respectively.

Further, the compound without molecular weight distribution in thepresent invention means the compound prepared aiming to controlmolecular weight in unity, and the compound may contain by-product inmanufacture, impurity or additives for a certain object. On thecontrary, a compound with molecular weight distribution means, acompound prepared by a manufacturing process in which the molecularweight cannot be controlled in unity such as, for example, amanufacturing process like polymerization, etc.

The foregoing styryl structure or distyryl structure, and the foregoingwithout having molecular weight distribution each improves crystallinityabout film properties in thin film-formation of an organic semiconductorlayer, resultantly enables to get superior films and enhanced currentcontrol characteristics.

It is preferable that the foregoing styryl derivative has at least theforegoing styryl structure and further, has a structure with arbitrarycombinations of a unit structure selected from a group consisting ofolefin, acetylene, aromatic hydrocarbon ring and aromatic heterocycle;wherein olefin and acetylene have bonding style between adjacent carbonatoms; and wherein aromatic hydrocarbon ring and aromatic heterocyclehave bonding style bonding at a position not between adjacent elements.It is more preferable that the styryl derivative has at least the styrylstructure and further, has a structure with arbitrary combinations ofolefin or acetylene and aromatic hydrocarbon ring or aromaticheterocycle that bonds each other alternately.

Similarly, it is preferable that the foregoing distyryl derivative hasat least the foregoing distyryl structure and further, has a structurewith arbitrary combinations of a unit structure selected from a groupconsisting of olefin, acetylene, aromatic hydrocarbon ring and aromaticheterocycle; wherein olefin and acetylene have bonding style betweenadjacent carbon atoms; and wherein aromatic hydrocarbon ring andaromatic heterocycle have bonding style bonding at a position notbetween adjacent elements. It is more preferable that the distyrylderivative has at least the distyryl structure and further, has astructure with arbitrary combinations of olefin or acetylene andaromatic hydrocarbon ring or aromatic heterocycle that bonds each otheralternately.

It is preferable that the above olefin has 2 to 8 carbon atoms. Examplesinclude ethylene, propylene, butene, pentene, hexene, heptene, octene,etc. More preferable examples include ethylene, propylene and butene.Particularly preferable example is ethylene.

It is preferable that the above aromatic hydrocarbon ring has 6 to 30carbon atoms. Examples include benzene, naphthalene, phenanthrene,anthracene, fluorene, perylene, pentacene and so on; each of which maybe substituted.

It is preferable that the above aromatic heterocycle has 1 to 30 carbonatoms. Examples include furan, thiophene, pyrrole, pyrazole, imidazole,triazole, tetrazole, oxazole, isoxazole, thiazole, thiadiazole,pyridine, pyrimidine, benzofuran, benzothiophene, indole, quinoline,carbazole, dibenzofuran, dibenzothiophene and so on; each of which maybe substituted.

Additionally, the phrase “olefin and acetylene have bonding stylebetween adjacent carbon atoms” means the following bonding styleswherein X and Y each independently represents olefin, acetylene,aromatic hydrocarbon ring or aromatic heterocycle:

X—CH═CH—Y

X—C≡C—Y

Further, the phrase “aromatic hydrocarbon ring and aromatic heterocyclehave bonding style bonding at a position not between adjacent elements”means the following bonding styles wherein X and Y each representsolefin, acetylene, aromatic hydrocarbon ring or aromatic heterocycle;and wherein a partial bonding structure is a single ring formula of5-member ring or 6-member ring, a double rings formula of 5-memberring—5-member ring or 6-member ring—6-member ring.

In the above bonding styles, A₁ to A₈ each independently represents acarbon atom or a nitrogen atom; B₁ to B₃ each independently represents acarbon atom, a nitrogen atom, an oxygen atom, a phosphorus atom or asulfur atom and forms an aromatic hydrocarbon ring or a aromaticheterocycle. The aromatic hydrocarbon ring or the aromatic heterocycleis capable of having an arbitrary substituent which may form a ringstructure with each other. When the arbitrary substituent of on thearomatic hydrocarbon ring or the aromatic heterocycle does not form aring structure, a number of the carbon atoms, the nitrogen atoms, theoxygen atoms, the phosphorus atoms or the sulfur atoms each possessed bya main chain composing the substituent is preferably 10 or less, andfurther preferably 6 or less.

The foregoing bonding styles enable to control an alignment of moleculesin compounds each other about the organic thin film transistor of thepresent invention, resultantly obtaining enhanced electric currentcontrol characteristics.

Furthermore, the phrase “a structure with arbitrary combinations ofolefin or acetylene and aromatic hydrocarbon ring or aromaticheterocycle that bonds each other alternately” means a followingrepeating structure when the olefin or acetylene is expressed as Z₁ andthe aromatic hydrocarbon ring or aromatic heterocycle is expressed asZ₂.

Z₂-(Z₁-Z₂)_(n)-Z₁-Z₂

wherein n preferably represents an integer of 0 to 20.

The foregoing bonding styles enable to strengthen a mutual actionbetween the compounds each other about the organic thin film transistorof the present invention, resultantly obtaining enhanced electriccurrent control characteristics.

In the present invention, it is preferable that the styryl derivative orthe distyryl derivative employed for the organic semiconductor layer isa styryl compound represented by a following general formula (a):

In the general formula (a), R₁ to R₅ and R₁₄ to R₁₈ each independentlyrepresents a hydrogen atom, a halogen atom, a cyano group, an alkylgroup having 1 to 30 carbon atoms, a haloalkyl group having 1 to 30carbon atoms, an alkoxyl group having 1 to 30 carbon atoms, ahaloalkoxyl group having 1 to 30 carbon atoms, an alkylthio group having1 to 30 carbon atoms, a haloalkylthio group having 1 to 30 carbon atoms,an alkylamino group having 1 to 30 carbon atoms, a dialkylamino grouphaving 2 to 60 carbon atoms and whose alkyl groups may bond each otherto form a ring structure containing a nitrogen atom, an alkylsulfonylgroup having 1 to 30 carbon atoms, a haloalkylsulfonyl group having 1 to30 carbon atoms, an aromatic hydrocarbon group having 6 to 60 carbonatoms, or an aromatic heterocyclic group having 1 to 60 carbon atoms,all of those may have a substituent.

In the general formula (a), R₆ to R₁₃ each independently represents ahydrogen atom, a halogen atom, a cyano group, an alkyl group having 1 to30 carbon atoms, a haloalkyl group having 1 to 30 carbon atoms, analkoxyl group having 1 to 30 carbon atoms, a haloalkoxyl group having 1to 30 carbon atoms, an alkylthio group having 1 to 30 carbon atoms, ahaloalkylthio group having 1 to 30 carbon atoms, an alkylamino grouphaving 1 to 30 carbon atoms, a dialkylamino group having 2 to 60 carbonatoms and whose alkyl groups may bond each other to form a ringstructure containing a nitrogen atom, an alkylsulfonyl group having 1 to30 carbon atoms, or a haloalkylsulfonyl group having 1 to 30 carbonatoms, all of those may have a substituent.

In the general formula (a), R₁₉ to R₂₂ each independently represents ahydrogen atom, a halogen atom, a cyano group, an alkyl group having 1 to6 carbon atoms, a haloalkyl group having 1 to 6 carbon atoms, an alkoxylgroup having 1 to 6 carbon atoms, a haloalkoxyl group having 1 to 6carbon atoms, an alkylthio group having 1 to 6 carbon atoms, ahaloalkylthio group having 1 to 6 carbon atoms, an alkylamino grouphaving 1 to 6 carbon atoms, a dialkylamino group having 2 to 12 carbonatoms and whose alkyl groups may bond each other to form a ringstructure containing a nitrogen atom, an alkylsulfonyl group having 1 to6 carbon atoms, or a haloalkylsulfonyl group having 1 to 6 carbon atoms,all of those may have a substituent.

When the number of carbon atoms of the above R₁₉ to R₂₂ is within arange of from 1 to 6, a film property in the occasion of the thinfilm-formation never degrades and any reduction in its electricfield-effect mobility or On/Off ratio never occurs.

In the general formula (a), l, m and n each independently represents aninteger of 0 to 10, preferably 0 to 5.

In the general formula (a), a sum of 1+m+n makes an integer of 0 to 20,preferably 1 to 20 and more preferably 1 to 10.

When the sum of l+m+n is 20 or smaller, a non-amorphous property of thefilm does not increase like polyphenylenevinylene high polymer, and anyreduction in its electric field-effect mobility or On/Off ratio neveroccurs. Further in the general formula (a), although a steric structureof olefin part may mix each other, it is preferable that an essentialcomponent has a steric structure wherein a conjugated main chain isdisposed in trans form.

In the general formula (a), it is preferable that R₁ to R₁₈ eachindependently represents a hydrogen atom, a halogen atom, a cyano group,an alkyl group having 1 to 30 carbon atoms, a haloalkyl group having 1to 30 carbon atoms, an alkoxyl group having 1 to 30 carbon atoms, ahaloalkoxyl group having 1 to 30 carbon atoms, an alkylthio group having1 to 30 carbon atoms, a haloalkylthio group having 1 to 30 carbon atoms,an alkylamino group having 1 to 30 carbon atoms, a dialkylamino grouphaving 2 to 60 carbon atoms and whose alkyl groups may bond each otherto form a ring structure containing a nitrogen atom, an alkylsulfonylgroup having 1 to 30 carbon atoms, or a haloalkylsulfonyl group having 1to 30 carbon atoms. Further, it is preferable that R₁₉ to R₂₂ eachindependently represents a hydrogen atom, a halogen atom, a cyano group,an alkyl group having 1 to 6 carbon atoms, a haloalkyl group having 1 to6 carbon atoms, an alkoxyl group having 1 to 6 carbon atoms, ahaloalkoxyl group having 1 to 6 carbon atoms, an alkylthio group having1 to 6 carbon atoms, a haloalkylthio group having 1 to 6 carbon atoms,an alkylamino group having 1 to 6 carbon atoms, a dialkylamino grouphaving 2 to 12 carbon atoms and whose alkyl groups may bond each otherto form a ring structure containing a nitrogen atom, an alkylsulfonylgroup having 1 to 6 carbon atoms, or a haloalkylsulfonyl group having 1to 6 carbon atoms, all of those may have a substituent.

In the general formula (a), it is further more preferable that R₆ to R₁₃and R₁₉ to R₂₂ each independently represents a hydrogen atom, a halogenatom, a cyano group, an alkyl group having 1 to 6 carbon atoms, or ahaloalkyl group having 1 to 6 carbon atoms.

Further in the general formula (a), R₃ and R₁₆ each independentlyrepresents a hydrogen atom, a halogen atom, a cyano group, an alkylgroup having 1 to 4 carbon atoms, a haloalkyl group having 1 to 30carbon atoms, an alkoxyl group having 1 to 30 carbon atoms, ahaloalkoxyl group having 1 to 30 carbon atoms, an alkylthio group having1 to 30 carbon atoms, a haloalkylthio group having 1 to 30 carbon atoms,an alkylamino group having 1 to 30 carbon atoms, a dialkylamino grouphaving 2 to 60 carbon atoms and whose alkyl groups may bond each otherto form a ring structure containing a nitrogen atom, an alkylsulfonylgroup having 1 to 30 carbon atoms, or a haloalkylsulfonyl group having 1to 30 carbon atoms.

The above preferable selection of R₃ and R₁₆, and arranging the abovenumber of carbon atoms to 30 or less will prevent a ratio of regularitycontrol position of R₃ and R₁₆ occupying in the styryl compound of thegeneral formula (a) from increasing too large, and will enable tocontrol the regularity of the film because a density ofphenylenevinylene skeleton contributing to current control, resultantlyenabling to get high electric field-effect mobility and high On/Offratio.

Still further in the general formula (a), it is preferable that R₁, R₂,R₄ to R₁₅ and R₁₇ to R₂₂ are all hydrogen atoms; at least one of R₃ orR₁₆ is an alkyl group having 1 to 3 carbon atoms, an alkoxyl grouphaving 1 to 3 carbon atoms, or a hydrogen atom. It is more preferablethat any one of R₁ to R₂₂ represents a fluorine atom, a trifluoromethylgroup or a pentafluoropropyl group.

Furthermore, the compound with the styryl structure or the distyrylstructure employed for the organic thin film transistor of the presentinvention has ambipolar transport property exhibiting p-type (holeconduction) and n-type (electron conduction), and the TFT can be drivenas p-type device or as n-type device according to a combination of thesource electrode and the drain electrode. However, an appropriateselection of the foregoing R₁ to R₂₂ in the general formula (a)depending on its necessity enables to enhance capabilities as p-type andn-type. In other words, an employment of a group with electron-acceptingproperty for R₁ to R₂₂ reduces a level of Lowest Unoccupied MolecularOrbital (LUMO) and enables to work as a n-type semiconductor. Preferableexamples of the group with electron-accepting property include hydrogenatom, halogen atom, cyano group, haloalkyl group having 1 to 30 carbonatoms, haloalkoxyl group having 1 to 30 carbon atoms, haloalkyl thiogroup having 1 to 30 carbon atoms and haloalkylsulfonyl group having 1to 30 carbon atoms. Further, an employment of a group withelectron-donating property for R₁ to R₂₂ enhances a level of HighestOccupied Molecular Orbital (HOMO) and enables to work as a p-typesemiconductor. Preferable examples of the group with electron-donatingproperty include hydrogen atom, alkyl group having 1 to 30 carbon atoms,alkoxyl group having 1 to 30 carbon atoms, alkylthio group having 1 to30 carbon atoms, alkylamino group having 1 to 30 carbon atoms anddialkylamino group having 2 to 60 carbon atoms, in which alkyl group maybond each other to form a ring structure.

Specific examples of the each group represented by R₁ to R₂₂ in thegeneral formula (a) will be explained below.

Examples of halogen atom include fluorine atom, chlorine atom, bromineatom and iodine atom.

Examples of the alkyl group include methyl group, ethyl group, propylgroup, isopropyl group, n-butyl group, s-butyl group, isobutyl group,t-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, n-octylgroup, etc. Examples of the haloalkyl group include chloromethyl group,1-chloroethyl group, 2-chloroethyl group, 2-chloro isobutyl group,1,2-dichloroethyl group, 1,3-dichloro isopropyl group,2,3-dichloro-t-butyl group, 1,2,3-trichloro propyl group, bromomethylgroup, 1-bromoethyl group, 2-bromoethyl group, 2-bromo isobutyl group,1,2-dibromo ethyl group, 1,3-dibromo isopropyl group,2,3-dibromo-t-butyl group, 1,2,3-tribromo propyl group, iodomethylgroup, 1-iodo ethyl group, 2-iodo ethyl group, 2-iodo isobutyl group,1,2-diiodo ethyl group, 1,3-diiodo isopropyl group, 2,3-diiodo-t-butylgroup, 1,2,3-triiodopropyl group, fluoromethyl group, 1-fluoromethylgroup, 2-fluoromethyl group, 2-fluoro isobutyl group, 1,2-diphloloethylgroup, difluoromethyl group, trifluoromethyl group, pentafluoro ethylgroup, perfluoro isopropyl group, perfluorobutyl group,perfluorocyclohexyl group, etc.

The above alkoxyl group is a group expressed by —OX¹, and examples of X¹are the same as those explained about the above alkyl group. The abovehaloalkoxyl group is a group expressed by —OX², and examples of X² arethe same as those explained about the above haloalkyl group.

The above alkylthio group is a group expressed by —SX¹, and examples ofX¹ are the same as those explained about the above alkyl group. Theabove haloalkoxyl group is a group expressed by —SX², and examples of X²are the same as those explained about the above haloalkyl group.

The above alkylamino group is a group expressed by —NHX¹, and the abovedialkylamino group is a group expressed by —NX¹X³, and examples of X¹and X³ are the same as explained about the above alkyl grouprespectively. Additionally, the alkyl group in the dialkylamino groupmay bond each other to form a ring structure having a nitrogen atom, andexamples of the ring structure include pyrrolidine, piperidine, etc.

The alkylsulfonyl group is a group expressed by —SO₂X¹, and examples ofX¹ are the same as those explained about the above alkyl group. Theabove haloalkylsulfonyl group is a group expressed by —SO₂X², andexamples of X² are the same as those explained about the above haloalkylgroup.

Examples of the above aromatic hydrocarbon group include phenyl group,naphthyl group, anthryl group, phenanthryl group, fluorenyl group,perilenyl group, pentacenyl group, etc. Examples of the aromaticheterocyclic group include furanyl group, thiophenyl group, pyrrolylgroup, pyrazolyl group, imidazolyl group, triazolyl group, tetrazolylgroup, oxazolyl group, isoxazolyl group, thiazolyl group, thiadiazolylgroup, pyridinyl group, pyrimidinyl group, benzofuranyl group,benzthiophenyl group, indolyl group, quinolinyl group, carbazolyl group,dibenzofuranyl group, dibenzothiophenyl group, etc.

Examples of a substituent that may be further substituted to each groupsrepresented in the foregoing general formula (a) include aromatichydrocarbon group, aromatic heterocyclic group, alkyl group, alkoxygroup, aralkyl group, aryloxy group, arylthio group, alkoxycarbonylgroup, amino group, halogen atom, cyano group, nitro group, hydroxylgroup, carboxyl group, etc.

Specific examples of the styryl derivative, distyryl derivative and thestyryl compound represented by the general formula (a) all employablefor the organic semiconductor layer of the organic thin film transistorof the present invention will be shown below, though not particularlylimited thereto.

The compound employed for the organic semiconductor layer of the organicthin film transistor of the present invention can be synthesized byvarious kinds of synthesizing processes. Namely, it can be synthesizedtaking descriptions in, for example, Organic Reactions Volume 14.3 (JohnWiley & Sons, Inc.), Organic Reactions Volume 25.2 (John Wiley & Sons,Inc.), Organic Reactions Volume 27.2 (John Wiley & Sons, Inc.), OrganicReactions Volume 50.1 (John Wiley & Sons, Inc.), etc., intoconsideration as references. Further, a steric structure at an olefinportion of the compound may be optionally arranged to a unit positionisomer utilizing a thermal reaction, a photo-reaction, an additionreaction and so on.

Regarding with an electronic device such as a transistor, an employmentof a material with high purity enables to get devices with high electricfield-effect mobility and with high On/Off ratio. Accordingly, anyoptional purification procedure such as column chromatography,re-crystallization, distillation, sublimation or so is preferablyconducted. A repeated conduct of the purification procedure or anycombination of the above procedures preferably enables to enhance thepurity. A repetition of at least twice of the sublimation purificationfurther as the last step of the purification is desirable. In accordancewith the above procedures, it is preferable to employ a material with apurity of 90% or greater measured by means of High performance liquidchromatography (HPLC). The employment of a material with high purityfurther preferably 95% or greater, particularly preferably 99% orgreater both measured by means of HPLC enhances electric field-effectmobility and On/Off ratio of the organic thin film transistor, whichenables to reveal an inherent performance of the material.

Following is a description about device structures regarding with theorganic thin film transistor of the present invention.

The above device structure is not specified as far as it is a thin filmtransistor comprising at least three terminals consisting of a gateelectrode, a source electrode and a drain electrode; an insulator layerand a novel organic semiconductor layer on a substrate, which controlsits electric current flowing between the source and the drain byapplying a electric voltage across the gate electrode. A devicestructure of public knowledge may be employable.

Among those, typical device structures of the organic thin filmtransistor are illustrated as Devices A, B, C and D in FIGS. 1 to 4. Asthe above description, a certain numbers of structures are known eachabout locations of the electrode, lamination sequence of layers and soon. The organic thin film transistor of the present invention has aField Effect Transistor (FET) structure. The organic thin filmtransistor comprises a novel organic semiconductor layer (an organiccompound layer), a source electrode and a drain electrode formedopposing each other with a predetermined space, and a gate electrodeformed with predetermined distances from the source electrode and thedrain electrode; and it has a structure which controls its electriccurrent flowing between the source and the drain by applying a electricvoltage across the gate electrode. In this occasion, the distancebetween the source electrode and the drain electrode is determineddependent on use purpose of the organic thin film transistor in thepresent invention, usually being 0.1 μm to 1 mm, preferably being 1 μmto 100 μm, and further preferably being 5 μm to 100 μm.

Among the Devices A, B, C and D, detailed explanation about anembodiment of Device B in FIG. 2 will be described below. An organicthin film transistor having a device structure illustrated as Device Bcomprises a gate electrode (layer) and an insulator layer in this orderon a substrate, further comprises a pair of a source electrode and adrain electrode formed with a predetermined distance each other on theinsulator layer, and a novel organic semiconductor layer formed overthem. The semiconductor layer forms a channel region and an electriccurrent flowing between the source electrode and the drain electrode iscontrolled by a voltage applied to the gate electrode resultantlycausing an On-Off operation.

Various kinds of structures are proposed about the organic thin filmtransistor except the above device structures in Devices A, B, C and Dof the present invention. Namely, the device structure is not specifiedto the above Devices A, B, C and D on the condition that it has amechanism revealing an effect in which an electric current flowingbetween the source electrode and the drain electrode is controlled by avoltage applied to the gate electrode resultantly causing an On-Offoperation or amplification. Examples of the device structure may includea top and bottom contact type organic thin film transistor (refer toFIG. 5) proposed in proceedings for the 49th Spring Meeting, The JapanSociety of Applied Physics, 27a-M-3 (March, 2002) by Yoshida et al. inNational Institute of Advanced Industrial Science and Technology, or avertical type organic thin film transistor (refer to FIG. 6) proposed onpage 1440 in IEEJ transactions 118-A (1998) by Kudo et al. of ChibaUniversity.

(Substrate)

A substrate in the organic thin film transistor of the present inventioncovers a role of supporting structures for the organic thin filmtransistor, and aside from glasses, inorganic compound such as metaloxide or metal nitride, plastics (PET, PES and PC) or metal substrate,these composite and lamination are employable as a material for thesubstrate. Further, in an occasion that the structures for the thin filmtransistor are supported enough by means of constituting element exceptthe substrate, the substrate may be absent. Furthermore, a silicon (Si)wafer is frequently used as a material for the substrate. In this case,Si may be used by itself as a gate electrode together with as thesubstrate. Still further, a surface of silicon (Si) substrate may beoxidized to form SiO₂, which may be utilized as an insulator layer. Inthis case, a metal layer employing gold (Au) and so on as an electrodefor connecting a lead wire is often formed in a manner as shown in FIG.8, on the Si substrate which is the gate electrode commonly used as thesubstrate.

(Electrode)

Examples of the material for the gate electrode, the source electrodeand the drain electrode in the thin film transistor of the presentinvention are not specified as far as they are electrically conductive,and include platinum (Pt), gold (Au), silver (Ag), nickel (Ni), chromium(Cr), copper (Cu), iron (Fe), tin (Sn), antimony-lead, tantalum (Ta),indium (In), palladium (Pd), tellurium (Te), rhenium (Re), iridium (Ir),aluminum (Al), ruthenium (Ru), germanium (Ge), molybdenum (Mo), tungsten(W), tin oxide, indium oxide tin (ITO), fluorine dope zinc oxide, zinc(Zn), carbon (C), graphite, glassy carbon, silver paste and carbonpaste, lithium (Li), beryllium (Be), sodium (Na), magnesium (Mg),potassium (K), calcium (Ca), scandium (Sc), titanium (Ti), manganese(Mn), zirconium (Zr), gallium (Ga), niobium (Nb), sodium-potassiumalloy, magnesium/copper mixture, magnesium/silver mixture,magnesium/aluminum mixture, magnesium/indium mixture, aluminum/aluminumoxide mixture, lithium/aluminum mixture, etc.

In the organic thin film transistor of the present invention, it ispreferable that the source electrode and the drain electrode are formedwith a use of a fluidic material for the electrode such as a solution, apaste, an ink, a dispersed solution or so each containing the foregoingelectro-conductive material. Particularly, it is preferable that thesource electrode and the drain electrode is formed with a use of anelectro-conductive polymer or the fluidic material for the electrodecontaining fine particles of a metal such as platinum, gold, silver orcopper. Further, it is preferable that the solvent or the dispersivemedium contains water in an amount of 60% by mass or greater, morepreferably in an amount of 90% by mass or greater in order to regulatedamage to the organic semiconductor. Regarding with a dispersedsubstance containing the fine particles of metal, for example, apublicly known electro-conductive paste may be employable, however, thedispersed substance containing the fine particles of metal havingparticle diameter in a range of usually from 0.5 nm to 50 nm, desirablyfrom 1 nm to 10 nm is preferable. Examples of the metal for preparingthe fine particles of metal include platinum, gold, silver, nickel,chromium, copper, iron, tin, hard lead, tantalum, indium, palladium,tellurium, rhenium, iridium, aluminum, ruthenium, germanium, molybdenum,tungsten, zinc, etc.

It is preferable that those fine particles of the metal are formed intothe electrode with a use of a dispersed mixture prepared by dispersingthem into a dispersant of water or an arbitrary organic solvent togetherwith a dispersion stabilizer essentially consisting of an organicmaterial. Examples of a process for preparing the dispersed mixture ofthe fine particle of metal include a physical generation process such asa vaporization process in gas, a sputtering process, a metallic vaporsynthesis process or so; and a chemical generation process thatgenerates the fine particles of the metal by reducing metal ions inliquid phase such as a colloid process, a co-precipitation process, etc.Among those, the dispersed mixtures prepared by the colloid processdisclosed in Japanese Unexamined Patent Application Laid-Open Nos. Hei11-76800, Hei 11-80647, Hei 11-319538, 2000-239853, etc; and by thevaporization process in gas disclosed in Japanese Unexamined PatentApplication Laid-Open Nos. 2001-254185, 2001-53028, 2001-35255,2000-124157, 2000-123634 and so on are preferable.

Molding the electrode with the use of the dispersed mixture of fineparticles of the metal, followed by drying the solvent and then,optionally heating the molded articles in an aimed pattern at atemperature within a range of from 100° C. to 300° C., preferably withina range of from 150° C. to 200° C. will thermally adhere fused fineparticles of the metal to a substrate and will form the aimed electrodepattern.

Further, it is preferable that publicly known electro-conductivepolymers whose conductance is improved by doping or so are employed asthe materials for the gate electrode, the source electrode and the drainelectrode. Preferable examples of the electro-conductive polymer includeelectro-conductive polyaniline, electro-conductive polypyrrole,electro-conductive polythiophene (complex of polyethylene dihydroxythiophene and polystyrene sulfonate), complex of polyethylene dihydroxythiophene (PEDOT) and polystyrene sulfonate, etc. Those materials reducethe contact resistances of the thin film transistors.

Among the above examples of the material for forming the sourceelectrode and the drain electrode, those materials having a smallelectric resistance at a contact surface with the organic semiconductorlayer are preferable. Because the electric resistance corresponds withelectric field-effect mobility in an occasion of preparing a currentcontrol device, it is necessary that the resistance is as small aspossible in order for getting a large mobility. Generally, theresistance depends on a relation between a work function of theelectrode material and an energy level of the organic semiconductorlayer.

Assuming that the work function of the electrode material (W) isrepresented by a, an ionization potential (Ip) of the organicsemiconductor layer is represented by b and the electron affinity (Af)of the organic semiconductor layer is represented by c; it is preferablethat they satisfy a following inequality. Additionally, each of a, b andc are all positive values on bases of their vacuum levels.

In an occasion of p-type organic thin film transistor, preferablyb−a<1.5 eV (Inequality (I)); more preferably b−a<1.0 eV. Althoughmaintaining the above inequality regarding with the organicsemiconductor layer enables to obtain the devices of favorableperformance, it is preferable that the work function of the electrodematerial is selected to be as large as possible. Namely, the workfunction is preferably 4.0 eV or greater, more preferably 4.2 eV orgreater.

The value of the work function about the metal may be selected from alist about effective metals having work function of 4.0 eV or greaterdescribed on Chemistry Manual; basic version II, page 493 (1983, thirdedition by the Chemical Society of Japan; published by Maruzen Co.,Ltd.). The list describes that the metal having great work function ismainly Ag (4.26, 4.52, 4.64, 4.74 eV), Al (4.06, 4.24, 4.41 eV), Au(5.1, 5.37, 5.47 eV), Be (4.98 eV), Bi (4.34 eV), Cd (4.08 eV), Co (5.0eV), Cu (4.65 eV), Fe (4.5, 4.67, 4.81 eV), Ga (4.3 eV), Hg (4.4 eV), Ir(5.42, 5.76 eV), Mn (4.1 eV), Mo (4.53, 4.55, 4.95 eV), Nb (4.02, 4.36,4.87 eV), Ni (5.04, 5.22, 5.35 eV), Os (5.93 eV), Pb (4.25 eV), Pt (5.64eV), Pd (5.55 eV), Re (4.72 eV), Ru (4.71 eV), Sb (4.55, 4.7 eV), Sn(4.42 eV), Ta (4.0, 4.15, 4.8 eV), Ti (4.33 eV), V (4.3 eV), W (4.47,4.63, 5.25 eV and Zr (4.05 eV). Among those, a noble metal such as Ag,Au, Cu or Pt; and Ni, Co, Os, Fe, Ga, Ir, Mn, Mo, Pd, Re, Ru, V or W ispreferable. Besides the metal, an electro-conductive polymer such asindium tin oxide (ITO), polyaniline or polyethylene dihydroxy thiophene(PEDOT) and polystyrene sulfonate (PSS); and carbon are preferable. Theelectrode material is not particularly specified even though itcomprises one or more kinds of those substances having high workfunction, as far as the work function satisfies the foregoing inequality(I).

In an occasion of n-type organic thin film transistor, preferablya−c<1.5 eV (Inequality (II)), more preferably a−c<1.0 eV. Althoughmaintaining the above inequality regarding with the organicsemiconductor layer enables to obtain the devices of favorableperformance, it is preferable that the work function of the electrodematerial is selected to be as small as possible. Namely, the workfunction is preferably 4.3 eV or smaller, more preferably 3.7 eV orsmaller.

Specific examples of the metal having small work function may beselected from a list about effective metals having work function of 4.3eV or smaller described on Chemistry Manual; basic version II, page 493(1983, third edition by the Chemical Society of Japan; published byMaruzen Co., Ltd.). The list describes that the metal having small workfunction include Ag (4.26 eV) Al (4.06, 4.28 eV), Ba (2.52 eV), Ca (2.9eV), Ce (2.9 eV), Cs (1.95 eV), Er (2.97 eV), Eu (2.5 eV), Gd (3.1 eV),Hf (3.9 eV), In (4.09 eV), K (2.28 eV), La (3.5 eV), Li (2.93 eV), Mg(3.66 eV), Na (2.36 eV), Nd (3.2 eV), Rb (4.25 eV), Sc (3.5 eV), Sm (2.7eV), Ta (4.0, 4.15 eV), Y (3.1 eV), Yb (2.6 eV), Zn (3.63 eV), etc.Among those, Ba, Ca, Cs, Er, Eu, Gd, Hf, K, La, Li, Mg, Na, Nd, Rb, Y,Yb or Zn is preferable. The electrode material is not particularlyspecified even though it comprises one or more kinds of those substanceshaving small work function, as far as the work function satisfies theforegoing inequality (II). However, because the metal having small workfunction will be easily degraded when it comes into contact withmoisture or oxygen among atmospheric air, it is preferable that themetal is optionally covered by a metal such as Ag or Au which is stableamong the air. Although a film-thickness necessary for covering themetal is 10 nm or more, and although a thicker film-thickness enables toprotect the metals from contacting with oxygen or water, it ispreferable that the film-thickness is determined up to 1 μm by reasonsof practice and productivity.

Regarding with a process for forming the electrode, they may be formedin accordance with, for example, a vapor deposition process, an electronbeam vapor deposition process, a sputtering process, an atmosphericpressure plasma process, an ion plating process, a chemical vapor phasevapor deposition process, an electro-deposition process, an electrolessplating process, a spin coating process, a printing process or an ofink-jet process, etc. Further, regarding with a patterning process whichmay be optionally carried out to the above resultant electro-conductivefilm, there is an electrode formation process with a use of a well-knownphotolithographic process or lift-off process, and an etching processafter forming a photoresist over a metal foil such as aluminum or copperby means of heat transfer, ink-jet, etc. Furthermore, the patterningprocess may be carried out directly by ink-jetting a solution or adispersed solution of the electro-conductive polymer, or a dispersedsolution containing the fine particles of the metal; or may be carriedout with a use of a lithograph or a laser abrasion against a coatedfilm. Moreover, the patterning may be also carried out using a printingprocess such as a relief printing, an intaglio printing, a planographicprinting, a screen printing and so on employing an electro-conductiveink comprising the electro-conductive polymer or the fine particles ofmetal or an electro-conductive paste.

The film-thickness of the resultant electrode is not particularlyspecified as far as the electrode is electrically conductive, however,the film-thickness falls within a range of preferably from 0.2 nm to 10μm, more preferably from 4 nm to 300 nm. When it falls within thepreferable range, any voltage drop will be prevented because a highelectric resistance dependent on a thin film-thickness never exists.Moreover, the film-thickness without exceeding the above range does notrequire long time for the film-formation, and further, even when otherlayers such as a protective layer and an organic semiconductor layer arelaminated on the electrode, the laminated layers will be smooth withoutsuffering from a generation of bumps.

Furthermore in the organic thin film transistor of the invention, forexample, a buffer layer may be sandwiched between the organicsemiconductor layer and a pair of the source electrode and the drainelectrode in order to improve carrier injection efficiency. Alkalimetals such as LiF, Li₂O, CsF, NaCO₃, KCl, MgF₂, CaCO₃ and so on usedfor a cathode of an organic electroluminescence device, or a compoundhaving alkaline earth metal ionic bond are preferable as the bufferlayer for the n-type organic thin film transistor.

(Insulator Layer)

Materials for the insulator layer in the organic thin film transistor ofthe present invention are not specified as far as they are electricallyinsulative and having ability of being formed into a thin film.Materials such as a metal oxide (including oxide of silicon), a metalnitride (including nitride of silicon), a polymer, and an organic lowmolecular compound each having an electric resistivity of 10 Ωcm orgreater at a room temperature may be employable and an inorganic oxidefilm having high dielectric constant is preferable as the material forthe insulator layer.

Examples of the inorganic oxide include silicon oxide, aluminum oxide,tantalum oxide, titanium oxide, tin oxide, vanadium oxide, bariumtitanate strontium, zirconic acid barium titanate, zirconic acid leadtitanate, lead titanate lanthanum, strontium titanate, barium titanate,barium fluoride magnesium, lanthanum oxide, fluorine oxide, magnesiumoxide, bismuth oxide, titanic acid bismuth, niobium oxide, strontiumtitanate bismuth, tantalic acid strontium bismuth, tantalum pentoxide,tantalic acid niobic acid bismuth, trioxide yttrium, and any combinationof those. Among those, silicon oxide, aluminum oxide, tantalum oxide andtitanium oxide are preferable. Further, inorganic nitride-based compoundsuch as silicon nitride (Si₃N₄, Si_(x)N_(y) (x, y>0)), aluminum nitrideand so on are preferable as the material for the insulator layer.

Further, the insulator layer may be formed with a precursor includingalkoxide metal. For example, applying a solution prepared by dissolvingthe precursor over the substrate, followed by chemical solutiontreatment or heat treatment will form the insulator layer.

The metal in the above alkoxide metal is selected from transition metal,lanthanoid or main group element, and specific examples include barium(Ba), strontium (Sr), titanium (Ti), bismuth (Bi), tantalum (Ta), zircon(Zr), iron (Fe), nickel (Ni), manganese (Mn), lead (Pb), lanthanum (La),lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs),francium (Fr) beryllium (Be) magnesium (Mg), calcium (Ca), niobium (Nb),thallium (Ti), mercury (Hg), copper (Cu), cobalt (Co), rhodium (Rh),scandium (Sc), yttrium (Y), etc. Further, examples of the alkoxide inthe above alkoxide metal are derived from alcohols including methanol,ethanol, propanol, isopropanol, butanol, isobutanol, etc.; or alkoxyalcohols including methoxyethanol, ethoxyethanol, propoxyethanol,butoxyethanol, pentoxyethanol, heptoxyethanol, methoxypropanol,ethoxypropanol, propoxypropanol, butoxypropanol, pentoxypropanol,heptoxypropanol, etc.

In the present invention, employing the above material for the insulatorlayer easily generates a depletion layer among the insulator layer andas a result, enables to reduce a threshold voltage of transistoroperation. Particularly, employing silicon nitrides such as Si₃N₄,Si_(x)N_(y), SiON_(x) (x, y>0) and so on among the above materials forthe insulator layer more easily generates a depletion layer among theinsulator layer and as a result, enables to further reduce a thresholdvoltage of transistor operation. Regarding with the insulator layeremploying an organic compound, polyimide, polyamide, polyester,polyacrylates, photo-curable resin of photo-radical polymerization-basedor photo cation polymerizaition-based, copolymer containing anacrylonitrile component, polyvinyl phenol, polyvinyl alcohol, novolacresin, cyanoethylpullulan, and so on are employable as the materials.

Additionally, wax, polyethylene, polychloropyrene,polyethyleneterephthalate, polyoxymethylene, polyvinylchloride,polyvinylidenefluoride, polymethylmethacrylate, polysulfon,polycarbonate, polyimidecyanoethylpullulan, poly(vinylphenol) (PVP),poly(methylmethacrylate) (PMMA), polycarbonate (PC), polystyrene (PS),polyolefin, polyacrylamide, poly(acrylic acid), novolac resin, resolresin, polyimide, polyxylylene, epoxide resin and further, highmolecular materials having high dielectric constant such as pullulan areemployable as the materials for the insulator layer.

Particularly preferable material for the insulator layer is an organiccompound having repellency. The repellency suppresses an interactionbetween the insulator layer and the organic semiconductor layer andraises crystallinity of the organic semiconductor layer taking advantageof the cohesiveness inherent in the organic semiconductor, whichimproves the device performance. Examples include polyparaxylylenederivative disclosed in Yasuda et al., Jpn. J. Appl. Phys. Vol. 42(2003) pp. 6614-6618; and the organic compound disclosed in Janos Vereset al., Chem. Mater., Vol. 16 (2004) pp. 4543-4555.

Further, when top gate structures illustrated in FIGS. 1 and 4 are used,employing the above organic compound as the material for the insulatorlayer is an effective skill for forming a film with little damageagainst the organic semiconductor layer.

The above insulator layer may be a mixed layer employing plural of theinorganic or organic compounds, or may be a laminated structure ofmultilayer. In this occasion, the device performance is controllable byoptionally mixing both the material having high dielectric constant andthe material having repellency, or by laminating multilayer.

Further, the insulator layer may be an anodic oxide film, or may containthe anodic oxide film as its constituent. It is preferable that asealing of anodic oxide coating is further applied to the anodic oxidefilm. The anodic oxide film is formed by anodically oxidizing an anodicoxidizable metal in accordance with a well known process. Aluminum ortantalum may be exemplified as the anodic oxidizable metal; and theprocess for anodic oxidation is not particularly specified permitting apublicly known process. The anodic oxidation treatment forms the oxidefilm. Any electrolytic solution capable of forming a porous oxide filmmay be employable as an electrolytic solution for the anodic oxidationtreatment. Generally, sulfuric acid, orthophosphoric acid, oxalate,chromic acid, boric acid, sulfamic acid, benzenesulfonic acid and so on;or a mixed acid made by combining 2 or more kinds of those acids orthose salt are employable as the electrolytic solution for the anodicoxidation treatment.

Although a condition for the electrolytic solution for the anodicoxidation treatment variously changes depending on the electrolyticsolution, it is generally appropriate that the concentration of theelectrolytic solution is within a range from 1 to 80% by mass, thetemperature of the electrolytic solution is within a range of from 5 to70° C., the current density is within a range of from 0.5 to 60 A/cm²,the voltage within a range of from 1 to 100 volt, and that the time ofelectrolytic treatment is within a range of from 10 seconds to 5minutes. A preferable anodic oxidation treatment is a process employingan aqueous solution of sulfuric acid, phosphoric acid or boric acid aselectrolytic solution; and treating with direct current, however,alternating current may be applicable. In this occasion, theconcentration of those acids is preferably within a range of from 5 to45% by mass, the temperature of the electrolytic solution is preferablywithin a range of from 20 to 50° C., the current density is preferablywithin a range of from 0.5 to 20 A/cm² and the time of electrolytictreatment is preferably within a range of from 20 to 250 seconds.

Regarding with a thickness of the insulator layer, a thin thickness willenhance an effectiveness voltage applied across the organicsemiconductor. Accordingly, it is possible to reduce both a drivingvoltage and a threshold voltage of the device itself, however on thecontrary, because a leak current between the source electrode and thegate electrode will increase, it is necessary to select an appropriatefilm thickness. The film thickness is preferably within a range from 10nm to 5 μm, more preferably within a range from 100 nm to 1 μm.

Further, an arbitrary orientation treatment may be carried out to aregion between the insulator layer and the organic semiconductor layer.A preferable example of the orientation treatment is a method conductinga water repelling treatment to a surface of the insulator layer therebyreducing a mutual action between the insulator layer and the organicsemiconductor layer and as a result, improving crystallinity of theorganic semiconductor layer. Specific example is a method that forms aself-assembling monolayer (SAM) by bringing materials for making theself-assembling orientation film like silane coupling agent such asoctadecyl trichlorosilane, trichloromethyl silazane, alkaneorthophosphoric acid, alkane sulfonic acid, alkane carboxylic acid or sointo contact with the surface of the insulator layer in the state of aliquid phase or a gas phase, followed by appropriately drying.Furthermore, exactly as practically used in the orientation of theliquid crystal device, a method of disposing a film consisting ofpolyimide or so over the surface of the insulator layer followed byconducting a rubbing treatment to the surface is also preferable.

Examples of the process for forming the above insulator layer includevacuum vapor deposition process, molecular beam epitaxial growthprocess, ion cluster beam process, low energy ion beam process, ionplating process, chemical vapor deposition (CVD) process, sputteringprocess; dry processes such as atmospheric pressure plasma processdescribed in Japanese Unexamined Patent Application Laid-Open Nos. Hei11-61406, Hei 11-133205, 2000-121804, 2000-147209 and 2000-185362; a wetprocess like a coating process such as a spray coating process, a spincoating process, a blade coating process, a dip coating process, acasting process, a roller coating process, a bar coating process, a diecoating process or so; and like patterning processes such as printing orink-jet, etc. They are applicable depending on the materials. The wetprocesses include a process of coating and drying a solution prepared bydispersing fine particles of inorganic oxide into an arbitrary organicsolvent or water with an optional use of an assistant dispersion agentlike surfactants; or a so-called sol-gel process of coating and dryingan oxide precursor, for example, solution of alkoxide article.

Although the film thickness of the organic semiconductor layer in theorganic thin film transistor of the present invention is notparticularly specified, it is usually within a range of from several nmto 1 μm, and preferably within a range of from 10 nm to 250 nm.

Further, although a process for forming the organic semiconductor layeris not particularly specified, various kinds of well-known processessuch as a molecular beam evaporation process (MBE process); a vacuumvapor deposition process; a chemical vapor deposition process; aprinting or coating process of a solution prepared by dissolving amaterial into a solvent, such as a dipping process, a spin coatingprocess, a casting process, a bar coating process, a roller coatingprocess, etc.; a baking process; electro-polymerization process; amolecular beam adhesion process; a self-assembly process from solution;are employable singly or in combination with the use of the abovematerials for the organic semiconductor layer.

Because improving the crystallinity of the organic semiconductor layerwill improve the field-effect mobility, it is desirable to maintain thetemperature of the substrate in the film-formation at high temperaturewhen a film-formation from the air phase (vapor deposition, sputtering,etc) is used. The temperature is preferably within a range of from 50 to250° C., more preferably within a range of from 70 to 150° C. Further,carrying out an annealing after the film-formation without concerningabout a process for film-formation is preferable because devices of highperformance are obtained. The annealing temperature is preferably withina range of from 50 to 200° C., more preferably within a range of from 70to 200° C. The annealing time is preferably within a range of from 10minutes to 12 hours, more preferably within a range of from 1 to 10hours.

In the present invention, one kind of material selected from a groupconsisting of the styryl derivative, distyryl derivative and a styrylcompound represented by the general formula (a) may be employed for theorganic semiconductor layer. Any combination of 2 or more kinds of thematerials among the above group, and plural mixed thin film ormultilayer each employing public known semiconductor such as pentaceneor thiophene may be also employed for the organic semiconductor layer.

Although a method for forming the organic thin film transistor of thepresent invention may be in accordance with any well-known methodwithout particularly specified, a serial successive steps for preparingthe device comprising placing a substrate, forming a gate electrode,forming an insulator layer, forming an organic semiconductor layer,forming a source electrode and forming a drain electrode all withoutcontacting with an atmosphere perfectly is preferable because it enablesto prevent disturbance against the device performance caused by moistureor oxygen among the atmosphere in contact. In an occasion that any oneof the above steps being unable to evade contacting with the atmosphere,it is desirable that a step of forming an organic semiconductor layerand all following steps never contact with the atmosphere and that theorganic semiconductor layer should be laminated after cleaning andactivating a surface over which the organic semiconductor layer is to belaminated, e.g., a surface over which a source electrode and a drainelectrode were partially laminated in the case of the foregoing DeviceB, by means of ultraviolet irradiation, ultraviolet/ozone irradiation,oxygen plasma, argon plasma, etc., just before forming the organicsemiconductor layer.

Further for example, considering about an influence of oxygen and watercontained in atmospheric air upon the organic semiconductor layer, a gasbarrier layer may be formed over entire or partial surface of outer faceof the organic transistor device. Regarding with a material for formingthe gas barrier layer, any substance usually employed in this technicalfield may be employable. Examples include polyvinyl alcohol,ethylene-vinyl alcohol copolymer, polyvinylchloride, polyvinylidenechloride, polychlorotrifloroethylene, etc. Furthermore, the inorganicsubstances having an insulating capability exemplified about theinsulator layer are also employable.

EXAMPLES

The present invention will be described more specifically with referenceto Examples and Synthesis Examples in the following.

Synthesis Example 1 Synthesis of Compound (1)

Under an atmospheric nitrogen gas and at a room temperature, adding amethanol solution of sodium methoxide (28%, 0.43 g) into a methanolsolution (5 milliliter) of benziltriphenylphosphoniumchloride (0.87 g)and after stirring the resultant solution for 1 hour, terephthalaldehyde (0.13 g) was added slowly to the reacted mixture solution andthe solution was stirred overnight. Adding water to the reacted mixturesolution, precipitated crystals were separated by filtration and washedwith the use of water and methanol. Dissolving the resultant crystalinto decalin with a temperature of 160° C. and adding iodine, theresultant solution was stirred for 2 hours. Then, a crystal generatedafter cooling with ice was separated by filtration, washed with the useof decalin and hexane, re-crystallized by toluene, purified bysublimation and as a result, the foregoing Compound (1) (0.18 g) wasobtained.

Example 1

An organic thin film transistor was prepared in accordance withfollowing steps. At first, after ultrasonically cleaning a glasssubstrate with the use of neutral detergent, pure water, acetone andethanol spending each 30 minutes respectively, a film of gold (Au)having a thickness of 40 nm was formed on the substrate in accordancewith a sputtering process resultantly making a gate electrode.Subsequently, the substrate was set on a film-forming zone of a thermalCVD equipment. On the other hand, polyparaxylene derivative[polyparaxylene chloride (parylene)] (trade name: diX-C; available fromDaisan KASEI CO., LTD.) as a material for an insulator layer in anamount of 250 mg was placed in a Petri dish and installed into anevaporation zone of the material. Vacuuming the thermal CVD equipment bymeans of a vacuum pump and after depressurizing it down to a pressure of5 Pa, both the evaporation zone and a polymerization zone were heated upto the temperature of 180° C. and 680° C. respectively. By leaving thematerial in the same situation for 2 hours, an insulator layer with athickness of 1 μm was formed over the gate electrode.

Then, the substrate was set in a vacuum vapor deposition apparatus(EX-400; manufactured by ULVAC Co.) and the above Compound (1) wasformed into a film of an organic semiconductor layer with a thickness of50 nm at a vapor deposition rate of 0.05 nm/second over the insulatorlayer. Subsequently, a source electrode and a drain electrode separatedwith a distance (channel length: L) of 75 μm between each other withoutcontacting each other were provided by forming a gold film having athickness of 50 nm through metal masking. Widths (channel widths Ws) ofboth the source electrode and the drain electrode were formed so as tobe 5 mm respectively. (refer to FIG. 7)

A gate voltage V_(G) of −40 V was applied upon the gate electrode of theabove organic thin film transistor and at the same time, a voltage wasapplied between the source and the drain resultantly feeding an electriccurrent. In this situation, carriers are induced within a channel region(between the source electrode and the drain electrode) of the organicsemiconductor layer, and the thin film transistor works as a p-typetransistor. As a result, a transistor characteristic as shown in FIG. 9was obtained. The On-Off ratio of the electric current between thesource electrode and the drain electrode within an electric currentsaturation domain was 4×10⁴. Further, an electric field-effect mobilityμ of holes was calculated by a following equation (A) and as a result,it was 7×10⁻³ cm²/Vs.

I _(D)=(W/2 L)Cμ(V _(G) −V _(T))²  (A)

In the equation (A), I_(D) represents an electric current between thesource electrode and the drain electrode, W represents a channel width,L represents a channel length, C represents an electric capacitance perunit area of the gate insulator layer, V_(T) represents a gate thresholdvoltage, and V_(G) represents a gate voltage.

Examples 2 to 5

Organic thin film transistors were prepared in similar manners asExample 1 except that compounds described in Table 1 were employedinstead of Compound (1). The prepared organic thin film transistors weredriven as p-type transistors under the gate voltage V_(G) of −40 V inthe same manner as Example 1. On-Off ratios of the electric currentbetween the source electrode and the drain electrode were measuredtogether with calculating the electric field-effect mobility μ of holes.The results are shown in Table 1.

Examples 6 and 7

Organic semiconductor layers were formed in similar manners as Example 1except that compounds described in Table 1 were employed as materialsfor the organic semiconductor layers instead of Compound (1).Subsequently, calcium (Ca) was vacuum vapor deposited as the source andthe drain electrode through metal masking at a deposition rate of 0.05nm/second instead of gold (Au) up to a thickness of 20 nm, followed byfurther vapor depositing silver (Ag) for covering calcium (Ca) at adeposition rate of 0.05 nm/second up to a thickness of 50 nm and as aresult, organic thin film transistors were prepared. The preparedorganic thin film transistors were driven as n-type transistors underthe gate voltage V_(G) of +40 V in the same manner as Example 1. On-Offratios of the electric current between the source electrode and thedrain electrode were measured together with calculating the electricfield-effect mobility μ of electrons. The results are shown in Table 1.

Example 8

An organic thin film transistor was prepared in accordance withfollowing steps. At first, a surface of a Si substrate (p-type;resistivity: 1 Ωcm; common use with a gate electrode) was oxidized inaccordance with a thermal oxidation process, forming a thermal oxidationfilm having a thickness of 300 nm over the substrate and as a result, aninsulator layer was provided. Further, after completely removing a SiO₂film formed over another surface of the substrate by means of dryetching, a chromium film with a thickness of 20 nm was formed inaccordance with a sputtering process, and further, a gold (Au) film witha thickness of 100 nm was formed in accordance with a sputteringprocess, resultantly taking out the films as an electrode. The substratewas ultrasonically cleaned with the use of neutral detergent, purewater, acetone and ethanol spending each 30 minutes respectively andafter further cleaning with the use of ozone, a self-assemblingmonolayer of octadecyltrichlorosilane was formed.

Then, the substrate was set in a vacuum vapor deposition apparatus(EX-400; manufactured by ULVAC Co.) and the above Compound (2) wasformed into a film of an organic semiconductor layer with a thickness of50 nm at a vapor deposition rate of 0.05 nm/second over the insulatorlayer (SiO₂). Subsequently, a source electrode and a drain electrodeseparated with a distance (channel length: L) of 75 μm between eachother without contacting each other were provided by forming a gold filmhaving a thickness of 50 nm through metal masking. An organic thin filmtransistor having a width (channel width W) of 5 mm between the sourceelectrode and the drain electrode was prepared (refer to FIG. 8).

The prepared organic thin film transistor was driven as p-typetransistor under the gate voltage V_(G) of −40 V in the same manner asExample 1. On-Off ratio of the electric current between the sourceelectrode and the drain electrode was measured together with calculatingthe electric field-effect mobility μ of holes. The results are shown inTable 1.

Example 9

A source electrode and a drain electrode separated with a distance(channel length: L) of 75 μm between each other without contacting eachother and having a width (channel width: W) of 5 mm were provided over aglass substrate having a thickness of 1 mm by forming a gold (Au) filmhaving a thickness of 50 nm through metal masking. Subsequently, theforegoing Compound (2) was formed into an organic semiconductor layerhaving a film thickness of 50 nm in accordance with vacuum vapordeposition process through another metal masking and then, an insulatorlayer of parylene having a film thickness of 1 μm was formed inaccordance with a thermal CVD process in a similar manner as Example 1.Finally, sputtering a gate electrode (Au) up to a thickness of 30 nmthrough another masking, an organic thin film transistor was prepared(refer to FIG. 1).

The prepared organic thin film transistor was driven as p-typetransistor under the gate voltage V_(G) of −40 V in the same manner asExample 1. On-Off ratio of the electric current between the sourceelectrode and the drain electrode was measured together with calculatingthe field-effect mobility μ of holes. The results are shown in Table 1.

Example 10

Cleaning of the substrate, film-forming of the gate electrode andformation of the insulator layer were carried out in the same manner asExample 1. Subsequently, dissolving 3% by mass of the Compound (30) intotoluene, the resultant solution was formed in accordance with a spincoating process to a film over the substrate after film-formation of theinsulator layer, and the film was dried under an atmosphere of anitrogen gas and at a temperature of 120° C. and as a result, an organicsemiconductor layer was film-formed. Subsequently, a source electrodeand a drain electrode separated without contacting each other wereprovided over the above film by forming a gold (Au) film having athickness of 50 nm through metal masking with a use of vacuum vapordeposition apparatus resultantly preparing an organic thin filmtransistor. (refer to FIG. 3)

The prepared organic thin film transistor were driven as p-typetransistor under the gate voltage V_(G) of −40 V in the same manner asExample 1. On-Off ratio of the electric current between the sourceelectrode and the drain electrode was measured together with calculatingthe electric field-effect mobility μ of holes. The results are shown inTable 1.

Comparative Example 1

An organic thin film transistor was prepared in the same manner asExample 10 except that polyparaphenylene vinylene (PPV) [molecularweight (Mn): 86000, molecular weight distribution (Mw/Mn=5.1)] was usedinstead of Compound (30).

The prepared organic thin film transistor was driven as p-typetransistor under the gate voltage V_(G) of −40 V in the same manner asExample 1. On-Off ratio of the electric current between the sourceelectrode and the drain electrode was measured together with calculatingthe electric field-effect mobility μ of holes. The results are shown inTable 1.

Comparative Example 2

Employing polyparaphenylenevinylene (PPV) as a material for the organicsemiconductor layer, steps until forming the organic semiconductor layerwere conducted completely in the same manner as Comparative Example 1.Then, in the same manner as Example 6, calcium (Ca) was vacuum vapordeposited as the source and the drain electrode through metal masking,followed by further vapor depositing silver (Ag) for covering calcium(Ca) and as a result, an organic thin film transistor was prepared.

The prepared organic thin film transistor was driven as n-typetransistor under the gate voltage V_(G) of 40 V in the same manner asExample 6. On-Off ratio of the electric current between the sourceelectrode and the drain electrode was measured together with calculatingthe electric field-effect mobility μ of electrons. The results are shownin Table 1.

TABLE 1 Compound in the Organic Field-Effect Semiconductor Type ofMobility On/Off Examples Layer Transistor (cm²/Vs) ratio 1 (1) p-type 7× 10⁻³ 4 × 10⁴ 2 (2) p-type 0.13 6 × 10⁵ 3 (40) p-type 0.2  5 × 10⁵ 4(43) p-type  0.028 1.2 × 10⁵   5 (92) p-type 6 × 10⁻³ 1 × 10⁵ 6 (16)n-type 0.01 1 × 10⁵ 7 (26) n-type 0.11 4 × 10⁵ 8 (2) p-type 0.15 4 × 10⁵9 (2) p-type 0.09 3 × 10⁵ 10  (30) p-type 0.01 4 × 10³ Comparative PPVp-type 1 × 10⁻⁵ 1 × 10³ Example 1 Comparative PPV n-type 1 × 10⁻⁴ 1 ×10³ Example 2

INDUSTRIAL APPLICABILITY

Exactly as explained above in detail, by employing the compound having aspecified structure with high mobility as a material of the organicsemiconductor layer, the organic thin film transistor of the presentinvention became to have a fast response speed (driving speed), andfurther, achieved a large On/Off ratio getting an enhanced performanceas a transistor.

1-16. (canceled)
 17. A thin film transistor comprising at least threeterminals consisting of a gate electrode, a source electrode and a drainelectrode; an insulator layer and an organic semiconductor layer on asubstrate, which controls its electric current flowing between thesource and the drain by applying a electric voltage across the gateelectrode, wherein the organic semiconductor layer comprises a styrylcompound represented by a following general formula (b):

wherein R₂₃ to R₂₇ each independently represents a hydrogen atom, ahalogen atom, a cyano group, an alkyl group having 1 to 30 carbon atoms,a haloalkyl group having 1 to 30 carbon atoms, an alkoxyl group having 1to 30 carbon atoms, a haloalkoxyl group having 1 to 30 carbon atoms, analkylthio group having 1 to 30 carbon atoms, a haloalkylthio grouphaving 1 to 30 carbon atoms, an alkylamino group having 1 to 30 carbonatoms, a dialkylamino group having 2 to 60 carbon atoms and whose alkylgroups may bond to each other to form a ring structure containing anitrogen atom, an alkylsulfonyl group having 1 to 30 carbon atoms, or ahaloalkylsulfonyl group having 1 to 30 carbon atoms, all of those mayhave a substituent and p, q and r each represents an integer of 0 to 10;and a sum of p+q+r makes an integer of 0 to 20.